Field of the Invention
The preferred embodiments are directed to scanning probe microscopy methods and apparatus, and more particularly, using an atomic force microscope (AFM) to collect topography, mechanical and electrical sample property data, preferably using peak force tapping mode (PFT mode) AFM and Kelvin Probe Force Microscopy (KPFM), respectively.
Description of Related Art
Scanning probe microscopes (SPMs), such as the atomic force microscope (AFM), are devices which typically employ a probe having a tip and which cause the tip to interact with the surface of a sample with low forces to characterize the surface down to atomic dimensions. Generally, the probe is introduced to a surface of a sample to detect changes in the characteristics of a sample. By providing relative scanning movement between the tip and the sample, surface characteristic data can be acquired over a particular region of the sample, and a corresponding map of the sample can be generated.
A typical AFM system is shown schematically in FIG. 11. An AFM 10 employs a probe device 12 including a probe 17 having a cantilever 15. A scanner 24 generates relative motion between the probe 17 and a sample 22 while the probe-sample interaction is measured. In this way, images or other measurements of the sample can be obtained. Scanner 24 is typically comprised of one or more actuators that usually generate motion in three mutually orthogonal directions (XYZ). Often, scanner 24 is a single integrated unit that includes one or more actuators to move either the sample or the probe in all three axes, for example, a piezoelectric tube actuator. Alternatively, the scanner may be a conceptual or physical combination of multiple separate actuators. Some AFMs separate the scanner into multiple components, for example an XY actuator that moves the sample and a separate Z-actuator that moves the probe. The instrument is thus capable of creating relative motion between the probe and the sample while measuring the topography or some other property of the sample as described, e.g., in Hansma et al. U.S. Pat. No. RE 34,489; Elings et al. U.S. Pat. No. 5,266,801; and Elings et al. U.S. Pat. No. 5,412,980.
In a common configuration, probe 17 is often coupled to an oscillating actuator or drive 16 that is used to drive probe 17 to oscillate at or near a resonant frequency of cantilever 15. Alternative arrangements measure the deflection, torsion, or other characteristic of cantilever 15. Probe 17 is often a microfabricated cantilever with an integrated tip 17.
Commonly, an electronic signal is applied from an AC signal source 18 under control of an SPM controller 20 to cause actuator 16 (or alternatively scanner 24) to drive the probe 17 to oscillate. The probe-sample interaction is typically controlled via feedback by controller 20. Notably, the actuator 16 may be coupled to the scanner 24 and probe 17 but may be formed integrally with the cantilever 15 of probe 17 as part of a self-actuated cantilever/probe.
As a selected probe 17 is oscillated, it is brought into contact with sample 22 as sample characteristics are monitored by detecting changes in one or more characteristics of the oscillation of probe 17, as described above. In this regard, a deflection detection apparatus 25 is typically employed to direct a beam towards the backside of probe 17, the beam then being reflected towards a detector 26, such as a four quadrant photodetector. The deflection detector is often an optical lever system such as described in Hansma et al. U.S. Pat. No. RE 34,489, but may be some other deflection detector such as strain gauges, capacitance sensors, etc. The sensing light source of apparatus 25 is typically a laser, often a visible or infrared laser diode. As the beam translates across detector 26, appropriate signals are processed by a signal processing block 28 (e.g., to determine the RMS deflection of probe 17). The interaction signal (e.g., deflection) is then transmitted to controller 20, which processes the signals to determine changes in the oscillation of probe 17. In general, controller 20 determines an error at Block 30, then generates control signals (e.g., using a PI gain control Block 32) to maintain a relatively constant interaction between the tip and sample (or deflection of the lever 15), typically to maintain a setpoint characteristic of the oscillation of probe 17. The control signals are typically amplified by a high voltage amplifier 34 prior to, for example, driving scanner 24. For example, controller 20 is often used to maintain the oscillation amplitude at a setpoint value, AS, to insure a generally constant force between the tip and sample. Alternatively, a setpoint phase or frequency may be used. Controller 20 is also referred to generally as feedback where the control effort is to maintain a constant target value defined by the setpoint.
A workstation 40 is also provided, in the controller 20 and/or in a separate controller or system of connected or stand-alone controllers, that receives the collected data from the controller 20 and manipulates the data obtained during scanning to perform data manipulation operating such as point selection, curve fitting, and distance determining operations. The workstation can store the resulting information in memory, use it for additional calculations, and/or display it on a suitable monitor.
AFMs may be designed to operate in a variety of modes, including contact mode and oscillating mode. Operation is accomplished by moving the sample and/or the probe assembly up and down relatively perpendicular to the surface of the sample in response to a deflection of the cantilever of the probe assembly as it is scanned across the surface. Scanning typically occurs in an “x-y” plane that is at least generally parallel to the surface of the sample, and the vertical movement occurs in the “z” direction that is perpendicular to the x-y plane. Note that many samples have roughness, curvature and tilt that deviate from a flat plane, hence the use of the term “generally parallel.” In this way, the data associated with this vertical motion can be stored and then used to construct an image of the sample surface corresponding to the sample characteristic being measured, e.g., surface topography. In one practical mode of AFM operation, known as TappingMode™ AFM (TappingMode™ is a trademark of the present assignee), the tip is oscillated at or near a resonant frequency of the associated cantilever of the probe, or harmonic thereof. A feedback loop attempts to keep the amplitude of this oscillation constant to minimize the “tracking force,” i.e., the force resulting from tip/sample interaction, typically by controlling tip-sample separation. Alternative feedback arrangements keep the phase or oscillation frequency constant. As in contact mode, these feedback signals are then collected, stored and used as data to characterize the sample.
Regardless of their mode of operation, AFMs can obtain resolution down to the atomic level on a wide variety of insulating or conductive surfaces in air, liquid or vacuum by using piezoelectric scanners, optical lever deflection detectors, and very small cantilevers fabricated using photolithographic techniques. Because of their resolution and versatility, AFMs are important measurement devices in many diverse fields ranging from semiconductor manufacturing to biological research. Note that “SPM” and the acronyms for the specific types of SPMs, may be used herein to refer to either the microscope apparatus or the associated technique, e.g., “atomic force microscopy.”
Kelvin-Probe Force Microscopy (KPFM), also known as Surface Potential Microscopy (SPoM), Surface Electric Potential Microscopy (SEPM), has been an important tool for electrical measurements using scanning probe microscopes (SPMs), such as AFMs, for many years.
Fundamentally, KPFM is a combination of atomic force microscopy (AFM) and Kelvin probe technique. Kelvin probe technique was designed to measure the contact potential difference (CPD) between an AFM probe and a sample surface when the two are brought close to one another. The CPD depends largely on the work function difference between the two materials. In this regard, the work function of a sample under test can be deduced if the work function of the probe is calibrated against a sample having a well-defined work function. Traditional Kelvin probe technique has a high sensitivity for potential measurements but offers poor spatial resolution. The invention of atomic force microscope (AFM) by Binnig et al. in 1986 (U.S. Pat. No. 4,724,318) opened the door to imaging solid sample surfaces of all kinds with nanometer to atomic resolution. Weaver et al. adapted Kelvin probe technique and combined it with AFM in 1991 (“High Resolution Atomic Force Microscopy Potentiometry”, Weaver et al., J. Vac. Sci. Technol. B Vol. 9, No. 3, May/June 1991, pp. 1559-1561); Nonnenmacher et al. coined the term Kelvin probe force microscopy shortly after (“Kelvin Probe Force Microscopy”, Nonnenmacher et al., Appl. Phys. Lett. Vol. 58, No. 25, June 1991, pp. 2921-2923). Thereafter, different AFM modes and KPFM detection schemes have appeared, and the various combinations of them have flourished the art.
Instead of measuring current as in Kelvin probe technique, KPFM is based on force measurement, employing the sensitive force detection capability in an AFM. The AFM probe and sample together is modeled as a parallel plate capacitor, the force between is thus:
      F    el    =            -              1        2              ⁢                  ∂        C                    ∂        z              ⁢                  (                  Δ          ⁢                                          ⁢          V                )            2      where Fel is the electric force, C the capacitance, and ΔV the voltage difference. ΔV can be the sum of CPD
      (          Δϕ      e        )    ,and the externally applied DC VDC and AC voltage VAC with frequency f:
      Δ    ⁢                  ⁢    V    =            V      DC        -          Δϕ      e        +                  V        AC            ⁢              sin        ⁡                  (                      2            ⁢                          π              ⁢              ft                                )                    
Various methods for implementing KPFM have been proposed, a discussion of some of which follows.
(i) The so-called amplitude-modulation method (AM-KPFM) in which an AC voltage between the probe and the sample excites a mechanical oscillation of the cantilever. AM-KPFM has been implemented in different variants, but common to all of them is that they minimize the electrostatic force by nullifying this electric force induced mechanical oscillation amplitude.
Pioneered by Weaver et al. in 1991, AM-KPFM includes applying an AC bias between the AFM probe and the sample, usually albeit not necessarily, at or near the mechanical resonance frequency f of the AFM cantilever to cause it to oscillate under the AC electric force there between. A DC bias voltage, also applied between the probe and the sample, is regulated by a KPFM feedback algorithm so that the oscillation at f stops. At this point, the AC electric force at frequency f is nullified, and the DC voltage applied equals exactly the CPD. As revealed by the following equation below,
      V    DC    =      Δϕ    e  when amplitude of the f term drops to 0. AM-KPFM is therefore a null-force technique.
                              F          el                =                                                            ∂                C                                            ∂                z                                      ⁢                          (                                                                    (                                                                  V                        DC                                            -                                              Δϕ                        e                                                              )                                    2                                +                                                      1                    2                                    ⁢                                                            V                      AC                                        2                                                              )                                +                                    DC        ⁢                                  ⁢        Term                                                                    ∂              C                                      ∂              z                                ⁢                      (                                          V                DC                            -                              Δϕ                e                                      )                    ⁢                      V            AC                    ⁢                      sin            ⁡                          (                              2                ⁢                                  π                  ⁢                  ft                                            )                                      +                            f        ⁢                                  ⁢        Term                                          1          4                ⁢                              ∂            C                                ∂            z                          ⁢                              V            AC                    2                ⁢                  cos          ⁡                      (                          4              ⁢                              π                ⁢                ft                                      )                                              2        ⁢        f        ⁢                                  ⁢        Term            
As shown in FIG. 12, an AM-KPFM 50 includes a) an AFM control block 52 configured to operate the AFM in either intermittent contact mode (AM-AFM) or non-contact mode (FM-AFM), and b) a KPFM control block 54. AM-KPFM 50 includes a probe 56 having a lever 58 supporting a tip 60 that is caused to interact with a sample 62 (note the charge distribution shown on the sample indicating electrical properties to be measured). In either AM-AFM mode or FM-AFM mode, an AC voltage is applied to, for example, the tapping piezo 64 by source 63 to cause the AFM probe 56 to oscillate at or near its resonance frequency f1. Deflection of probe 106 during operation is measured by directing a laser beam from source 68 toward the backside of lever 58 and toward detector 70. The deflection signal from detector 70 is transmitted to signal processing block 72 of AFM control block 52 to determine an appropriate control signal to maintain probe-sample interaction at a setpoint. This feedback control signal (together with a scanning control signal provided in block 74) is transmitted to an actuator 66 (e.g., an X-Y-Z piezoelectric tube) to appropriately position the probe 56 supported thereby.
AM-KPFM control block 54 includes a source 78 which delivers an AC bias at a second frequency f2, is applied between the probe and the sample, giving rise to an alternating electric force between probe 56 and sample 62, causing the probe to oscillate at this frequency as well. In operation, the detected oscillation signal from detector 70 is transmitted to a lock-in amplifier 76 of control block 54 for comparison to the AC bias output by source 78. An AM-KPFM feedback block 80 generates an appropriate control voltage signal which is added to the AC bias at block 82. Control block 54 continues to adjust the DC bias so that probe oscillation at f2 drops to 0. At this point Vdc equals the contact potential difference (CPD) between sample 62 and probe 56.
(ii) The so-called frequency-modulation method (FM-KPFM) detects the resonance frequency shift Δf induced by the bias voltage applied between tip and sample. FM-KPFM is sensitive to the electric force gradient, which is much more confined to the tip front end than electric force. Hence, for the FM method higher lateral resolution than for the force-sensitive AM method is expected.
Frequency-modulation KPFM (FM-KPFM) was introduced in 1991 by Nonnenmacher et al, and was perfected under ultrahigh vacuum in 1998 by Kitamura et al. (“High-resolution Imaging of Contact Potential Difference with Ultrahigh Vacuum Noncontact Atomic Force Microscope”, Kitamura et al., Appl. Phys. Lett. Vol. 72, No. 24, June 1998, pp. 3154-3156; and U.S. Pat. No. 6,073,485). Typically, the cantilever is mechanically driven by a tapping piezo at or near the resonance frequency of the cantilever f, and an AC bias is applied between the probe and the sample with a frequency fm usually much lower than the fundamental probe resonant frequency. The AC bias modulates the electric force gradient between the probe and the sample, thus periodically changing the effective spring constant of the cantilever; this causes the resonance frequency to shift periodically, that is, to modulate, at fm and 2fm. A DC voltage is adjusted by the KPFM feedback algorithm so that the frequency modulation at fm stops. At this point, the electric force gradient is nullified, and the DC voltage applied measures the CPD,
      V    ⁢    dc    =      Δϕ    e  as expressed in the following equation.
                                          ∂                          F              el                                            ∂            z                          =                                            1              2                        ⁢                                                            ∂                  2                                ⁢                C                                            ∂                                  z                  2                                                      ⁢                          (                                                                    (                                                                  V                        DC                                            -                                              Δϕ                        e                                                              )                                    2                                +                                                      1                    2                                    ⁢                                                            V                      AC                                        2                                                              )                                +                                    DC        ⁢                                  ⁢        Term                                                                                    ∂                2                            ⁢              C                                      ∂                              z                2                                              ⁢                      (                                          V                DC                            -                              Δϕ                e                                      )                    ⁢                      V            AC                    ⁢                      sin            ⁡                          (                              2                ⁢                                  f                  m                                ⁢                t                            )                                      +                                      f          m                ⁢                                  ⁢        Term                                          1          4                ⁢                                            ∂              2                        ⁢            C                                ∂                          z              2                                      ⁢                              V            AC                    2                ⁢                  cos          ⁡                      (                          4              ⁢              π              ⁢                                                          ⁢                              f                m                            ⁢              t                        )                                              2        ⁢                  f          m                ⁢                                  ⁢        Term            
FM-KPFM is a null-force-gradient technique. As shown in FIG. 13, an FM-KPFM 100 includes a) an AFM control block 102 configured to operate the AFM in either intermittent contact mode (AM-AFM) or non-contact mode (FM-AFM), and b) a KPFM control block 104. AFM 102 includes a probe 106 having a lever 108 supporting a tip 110 that is caused to interact with a sample 112 (note the charge distribution shown on the sample indicating electrical properties to be measured). In either AM-AFM mode or FM-AFM mode, an AC voltage is applied to the tapping piezo 114 by source 113 to cause the AFM cantilever to oscillate at or near its resonance frequency f1. Deflection of probe 106 during operation is measured by directing a laser beam from source 115 toward the backside of lever 108 and toward detector 117. The deflection signal from detector 117 is transmitted to signal processing block 118 of AFM control block 102 to determine an appropriate control signal to maintain probe-sample interaction at a setpoint. This feedback control signal (together with a scanning control signal provided in block 120) is transmitted to an actuator 116 (e.g., an X-Y-Z piezoelectric tube) to appropriately position the probe supported thereby.
FM-KPFM control block 104 includes a source 128 that provides an AC bias at frequency f2, usually a few kHz, applied between the probe and the sample, giving rise to an alternating electric force gradient between probe 106 and sample 112. This force gradient will cause the probe resonant frequency to modulate, manifested as sidebands at f1±nf2 which are used for KPFM feedback. The sideband frequencies are known (Block 124) and input to lock-in amplifier 122 for comparison to the output signal of detector 117. Preferably, a KPFM feedback block 126 operates to continuously adjust the DC bias (which is added to the AC bias at block 130) so that the probe response at the side bands (f1±f2) drops to 0. When doing so, the Vdc equals the contact potential difference (CPD) between sample 112 and probe 106 providing one of the electrical properties of the sample at that location.
One of the challenges with KPFM, FM-KPFM mode in particular, is that a low spring constant and high cantilever Q is desirable for making sensitive high resolution measurements. However, though one of the major advantages of SPM over other high-resolution microscopes (such as SEM) is operation in air, SPM limits possible Q values. Therefore, to achieve sensitive KPFM measurement capability, it is known that using a probe with the lowest possible spring constant can offset limited Q values of the SPM (theoretical explanation is detailed in the probe design section of this invention). However, for practical reasons (e.g., to help prevent the probe tip from sticking to the sample as it makes intermittent contact therewith), TappingMode requires use of probes having relatively high spring constants for reliable operation; and not too high a Q value to attain a bandwidth that allows a reasonably fast scan rate. KPFM sensitivity is thereby necessarily limited. Contact mode permits the use of levers having lower spring constants but is generally known to be one of the most destructive SPM techniques and is therefore limited with respect to which the types of samples with which it can be used.
KPFM accuracy and resolution can also be limited by any one or more of the following: tip wear, tip contamination (need stable tip work function), metal degradation particularly over the apex, parasitic capacitance (compromise lateral resolution), parasitic electrochemistry, unintentional charge dissipation from the sample, etc. Applicants realized a probe design that overcomes these drawbacks would be beneficial.
In the end, notwithstanding the broad application and capabilities of AFM and KPFM, limitations with each have remained. Sensitivity is the primary limitation of KPFM. Nanometer scale sample features have many interesting material properties and AFM has been one of the major tools to characterize them. However, while AFM is reliable in providing multidimensional information with very high resolution and has gained broad acceptance as the tool of choice for many applications in imaging (e.g., topography), AFM has not been as successful with respect to quantitative mechanical property characterization.
Another fundamental limitation of current KPFM is that it is integrated with Tapping Mode (or intermittent contact mode or AC mode). Stability of the tapping mode critically depends on the spring constant of the cantilever probe k, where the common value of k is about 40 N/m and can be reduced to 5 N/m with marginal performance. The sensitivity factor of the KPFM detection is defined by Q/k, where Q is the mechanical quality factor of the cantilever. Given a typical “Q” of 200 on surface, normal KPFM usually have a sensitivity factor from 5-40. Using probes having a much lower spring constant is therefore desirable; however, low spring constant probes are incompatible with the requirements of Tapping Mode feedback stability.
More particularly, conventional AFM has been known for its inability to simultaneously acquire both high-resolution images and quantitative mechanical property information (e.g., elasticity, plasticity, and adhesion). Measuring mechanical properties with an AFM experimental setup is possible, but most known methods and systems rely on collecting force curves corresponding to the local tip-sample interaction, an extremely slow process.
Additionally, current KPFM measurement systems are subject to large variations in the surface potential value, due primarily to changes at the probe apex during imaging. For example, the surface potential of gold (Au) is around 800 mV. A first concern is drift during measurement. Drift can be substantial, often exceeding hundreds of mV, thereby making accurate measurements using AFM impractical. In addition, the measured surface potential varies when a probe is replaced or used for an extended period of time, and can also vary from system to system. These changes are typically caused by uncertainty and variation of the conductive coating on the AFM probe; in particular, the crystal structure of the apex of the tip is poorly defined and changes from probe to probe.
Generally, as shown in FIG. 14, an AFM probe 600 consists of two parts, a cantilever 602 which is sensitive to the forces between the tip and the sample, and a tip 604. The tip 604 includes a body 606 having a base 608 that connects to or is otherwise support by cantilever 602 and an apex 610 having a radius in nanometer range. Apex 610 is the part of the probe that interacts with the sample, with the resolution of the AFM substantially defined by the radii of the apex. Most KPFM measurement apparatus and methods utilize either i) a coated probe (conducting), where the conducting material at apex 610 is poorly defined due primarily to inherent imperfections in the coating process, or ii) an etched metal wire, the mechanical properties of which are poorly controlled during fabrication, as understood in the art. In either case, the KPFM measurement is compromised.
In sum, the microscopy field has been left wanting a more comprehensive instrument capable of fast, high sensitivity electrical, topography, and mechanical sample property measurement. Ideally, the tool would be capable of associating the measured electrical properties with the corresponding mechanical properties of the sample at each data acquisition location.